In general, in internal combustion engines, since a fuel injection amount is set based on intake air volume, it is required to always measure the intake air volume.
In particular, in an independent air intake type of internal combustion engine such as a multi-slot type and single cylinder type, since there is no air intake chamber upstream of an air intake pipe, it is difficult to measure the intake air volume.
Therefore, it has been performed that the intake air volume is estimated using the number of rotations of the internal combustion engine and a throttle open angle as parameters.
However, in such a method, there occurs an inconvenience that the actual intake air volume departs from the state of operation of the internal combustion engine that is determined based on the number of rotations and the throttle open angle.
In order to overcome such an inconvenience, a method has been proposed of detecting a pressure inside an air intake pipe using an intake air pressure sensor and based on the intake air pressure, estimating the intake air volume.
When applying such an estimation method to the independent air intake type of internal combustion engine, since there is no air intake chamber as described above, a large intake air pressure pulsation occurs, which causes such an inconvenience such that the intake air volume cannot be measured accurately.
That is, such a large pulsation occurs, in particular, when a large negative pressure is generated in the intake at the time of medium load and then the pressure returns to the atmospheric pressure.
In order to estimate the intake air volume, it is required to unify varying intake air pressures, and therefore, the varying intake air pressures are averaged.
However, when averaging the large intake air pressure pulsation as described above, since pressures near the atmospheric pressure are also included to average, it is not possible to estimate an accurate intake air volume, and such averaging is not effective in estimating the intake air volume.
Further, in order to overcome such an inconvenience, a method has been performed of extracting intake air pressures at specific timing or in a specific range with respect to a crank angle signal, and averaging the specific intake air pressures.
For example, as disclosed in JP H11-62691, intake air pressures near the bottom dead center of the piston are averaged.
However, in the method for estimating the intake air volume using the value obtained by averaging intake air pressures in such a specific range, it is required to set the timing of detecting the intake air pressure using the crank angle signal, resulting in a problem that the control is complicated.
Further, when an intake air pressure sensor is provided at a position some distance from an air intake pipe (for example, in an engine control unit), the air intake pipe and intake air pressure sensor are communicated using a connecting pipe.
Therefore, a timewise delay occurs between an actual variation in pressure inside the air intake pipe and a detected variation in pressure, or the detected variation in pressure is smoothed as compared to the actual variation in pressure i.e. so-called rounding occurs, depending on the capacity of the connecting pipe, thereby making it difficult to sample accurate intake air pressures at specific crank timings.
In view of the above-mentioned conventional problems, it is an object of the present invention to provide a method for measuring intake air volume in an internal combustion engine, which enables estimation of intake air volume with high precision, by setting the relationship between a detected intake air pressure and intake air volume to be more linear independently of crank timing, in estimating the intake air volume based on the intake air pressure.